As is known, jammers emitting radar energy can degrade target detection and tracking performance of radar systems. The jammers can either be active jammers having the purpose of degrading radar performance, as in a battlefield, or the jammers can be non-combative radar emitters, for example, an aircraft in the vicinity of the radar system that has an on-board active radar system.
Adaptive radar arrays are used in conjunction with adaptive beamforming within radar systems to reduce the impact of jammers on radar system detection and tracking performance. A conventional radar array, also referred to herein as a radar antenna, can have thousands of individual transceiver elements, each capable of transmitting and receiving radar energy with a generally omnidirectional spatial pattern. The elements are combined, either in a transmit mode or in a receive mode or both, resulting in one or more transmit radar beams and one or more receive radar beams having spatial directivity. The receive radar beams and the transmit radar beams can be the same or different. In the receive mode, it is often desirable to adaptively change the resulting receive beampattern in response to a jammer, for example, by pointing a beampattern null at the jammer, in order to reduce the affect of the jammer on the radar system detection and tracking performance.
In order to form a receive mode radar beam, often the elements of a radar array are divided into subarrays, each corresponding to a subset of the total number of array elements. Within each subarray, subarray elements can be statically combined to provide subarray beampatterns (with a desired pointing direction for that dwell) having a static geometry. Outputs of the subarrays, i.e., the subarray beampatterns, can be dynamically combined to generate a receive beam that can be dynamically modified, for example, having a main beam directed toward a target of interest while reducing sidelobe levels in a direction of a noise source.
The dynamic combining of the subarray outputs is often performed digitally, wherein the outputs of subarrays are digitized and complex adaptive weighting factors are applied. In this way, the receive beam can be adapted in direction and in shape, including receive beam nulls that can also be adapted in direction and shape, by way of complex adaptive weighting factors applied to the outputs of the subarrays in the combining process.
Radar systems are known that can determine range and bearing of a target from each individual transmitted and received radar pulse. Angle estimation techniques (bearing) can include monopulse measurements and various most likely angle estimators. Many conventional radar systems simultaneously form a plurality of receive beams, and in particular, a monopulse radar simultaneously receives a “sum” beam and one or more “difference” beams in receive mode. The sum beam will be understood to be a radar beam having a maximum response axis generally in a direction of a target. The difference beam will be understood to be a beam having a null generally in the direction of the target. As described above, a direction of the maximum response axis of the sum beam and a direction of the null of the difference beam can be influenced by complex adaptive weighing factors applied to the digitized outputs of the subarrays. Likewise, for a most likely angle estimator, the complex adaptive weighting factors are used to modify the effective receive beam shape to maximize energy received from the target and minimize energy received from noise sources.
When adapting a receive beampattern, it is known that grating lobes and grating nulls can be generated along with a desired receive beam. Grating lobes tend to degrade radar system detection and tracking performance. It is known that grating lobes and grating nulls are influenced by a variety of factors, including, but not limited to, array element relative spacings and positions within each subarray and subarray relative spacings and positions. It is also known that the affect of grating lobes and grating nulls can be reduced by use of irregularly shaped subarrays and irregular subarray relative spacings.
Referring to FIG. 1, a conventional adaptive radar array 10 is described by Nickel, U., A Corrected Monopulse Estimation Method for Adaptive Arrays, IEEE International Conference on Radar, page 327, FIG. 5, Brighton, 1992.
The adaptive array 10 has elements identified as solid triangles, which are grouped as subarrays identified as polygons. Each subarray has a different shape, number of elements, and position on the plane of the radar array.
Taking subarray 12 as representative of the other subarrays, (though the other subarrays have different shapes, numbers of elements, and element positions), the subarray 12 has a plurality of elements, for example element 14, and a resulting phase center 16. Each one of the subarrays has a respective phase center, and the plurality of phase centers can be irregularly spaced.
It will be appreciated that the non-symmetrical geometry of the adaptive radar array 10 results in an expensive radar array. Each subarray, having a different geometry, is associated with combining circuitry that performs a static combination of respective subarray elements. The combining circuitry, therefore, can be physically different for each subarray. Thus, a variety of versions of combining circuitry must be designed, built, and maintained for the adaptive radar array 10, resulting in a adaptive array that is costly and difficult to manufacture.
The adaptive radar array 10 has a geometry representative of but one of a variety of conventional adaptive radar arrays. However, each conventional adaptive radar array geometry has a configuration with few or no repetitive geometrical characteristics, and therefore, suffers from the same cost and manufacturing disadvantages.